This invention relates to current control systems for switched reluctance motors and, more particularly, to a current control system for a switched reluctance motor that eliminates the unwanted effects of back EMF.
It is well known in the art of switched reluctance motors (SRM) to control such motors by controlling the current supplied to the motor. Two main types of current regulators have been used to control the motor current, hysteretic current regulators and low performance proportional and integral (PI) current regulators.
The hysteretic regulators are the most common, but have several drawbacks. They are typically variable frequency, which can cause electromagnetic interference (EMI) or acoustic noise problems. To prevent such problems, the maximum switching frequency is limited to prevent excessive switching losses in the inverter. Another drawback is that the hysteretic regulator is usually implemented using discrete analog circuitry which increases part count, cost and reduces reliability compared to a fully digital microcontroller or digital signal processor (DSP) based implementation. The hysteretic regulator may be implemented digitally, with either a microcontroller/DSP or high speed digital logic circuits. However, these implementations require a high sample rate to achieve adequate performance, which increases the cost of the hardware.
Low performance PI current regulators have also been used, in either digital or analog implementations. Their advantages over hysteretic regulators arc their fixed switching frequency and ease of digital implementation. There are several drawbacks when using a PI current regulator with a SRM. It is difficult to obtain a good performance of the PI current regulator because of the non-sinusoidal nature of the SRM, which prevents the use of a synchronous frame to regulate the current. Further, the highly non-linear nature of the machine operation makes the design of the PI regulator difficult. The back EMF of the motor, which is non-sinusoidal and non-linear with the rotor position and current, acts as a disturbance to the current loop. To adequately reject the back EMF disturbance, the PI regulator gains must be increased which may cause stability problems.
However, a third type of controller has been designed to overcome this problem. It is called a hybrid current controller. The controller performs as a fixed frequency PI regulator when the current error is small and transforms to hysteretic control when the error exceeds some predefined threshold. The controller implements a simplified back EMF decoupling technique which attempts to eliminate the effects of the back EMF of the motor. A drawback of this combination of hybrid controller and decoupling technique is that it may be only adequate as long as the hysteretic controller is there to catch any errors that the partially decoupled PI regulator could not eliminate.
Another drawback, as mentioned above, is that to implement the hysteretic controller digitally, a high sample rate is required to achieve adequate performance. Further, the back EMF decoupling technique uses a simple model of the back EMF which may not be adequate to model most SR motors. The SR motor must be operated deep into saturation in order to achieve maximal torque output for a given machine size and weight.
The back EMF calculation used in the technique models the machine inductance as a piecewise linear inductance. The derivative of inductance with respect to rotor position is then approximated as a constant in the regions of increasing and decreasing inductance, and zero elsewhere. The result is that between the unaligned and aligned positions of the rotor, a single value of ∂L/∂xcex8 is used. The same value is used in the motor torque and braking torque regions, but with opposite signs. Finally, the back EMF is calculated as:   e  =                    ∂        L                    ∂        θ              ·    ω    ·    i  
This simplified back EMF calculation may result in significant errors if used on highly saturated machines. It is then up to the hysteretic controller to catch and control these errors. It would be preferable to have a complete stand alone decoupled PI current regulator that could provide sufficient current regulation without relying on a hysteretic controller to compensate for deficiencies in the PI current regulator.
The present invention provides a current control system for a switched reluctance motor that allows the use of a fixed frequency PI current regulator while achieving a good performance and current command tracking. The control system includes a current regulator that receives a current reference signal indicative of the desired output of the motor and a motor current signal indicative of the actual current in at least one phase winding of the motor. The regulator produces an output signal in response to the difference between the current reference signal and the motor current signal. The output of the PI regulator is a duty cycle command. A back EMF decoupler provides a back EMF signal indicative of the back EMF created by the motor and adds the back EMF signal to the PI regulator output to provide a back EMF decoupled duty cycle command signal which is fed to an inverter circuit. The inverter circuit switches on and off the voltage applied to at least one phase winding of the motor in accordance with the back EMF decoupled duty cycle command signal to regulate the motor current and operate the motor.
In one embodiment of the present invention, the back EMF signal is determined as a function of current, rotor position and rotor speed. The back EMF signal has a duty cycle which varies depending on the type of chopping used. For soft chopping, the back EMF signal has a duty cycle equal to a calculated back EMF divided by a DC voltage supplied to the inverter circuit. For hard chopping, the back EMF signal has a duty cycle equal to a sum of a calculated back EMF and the DC voltage, the sum divided by twice the DC voltage. The calculated back EMF is equal to the partial derivative taken at constant current of the phase flux linkage with respect to the rotor position times the rotor speed.
The control system of the present invention eliminates the effects of the back EMF of the motor on the PI regulator by adding the back EMF to the current loop. This reduces the bandwidth requirement of the current regulator to achieve a specified performance or command tracking.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.